Criminals are reoffending to pay off drug debts from their time behind bars, new research shows.

It reveals prisoners' increased tolerance for the drug known as Spice led to some of them smoking between five and eight grams daily.

With Spice selling for up to £100 a gram in prison, some quickly got into debt and turned to crime to pay it off, according to the report by HM Inspectorate of Probation.

"Those in debt were often involved in violent incidents where threats were made to family members, with some stating that they offended to repay the debts they had built up in prison," the report said.

Spice and other new psychoactive substances (NPS), formerly known as legal highs which are cheap and difficult to detect in tests, have been found to have fuelled a recent surge in prison violence.

One probation officer said: "People are crazy when they are under the influence, one confused me for a fire hose when he was under the influence."

There were also concerns in the report about the risks to children when in contact with offenders using the drug.

It said: "We saw case records where responsible officers were aware that service users who reported using NPS daily were on their way to see their children.

"Such safeguarding concerns had not been sufficiently analysed."

Chief Inspector of Probation Dame Glenys Stacey said: "We found that probation staff and even some substance misuse service staff had a low level of awareness of NPS.

"Probation staff did not have structured, in-depth training about NPS and how to deal with dependency, and lacked the confidence and knowledge to quantify the problem and to address it."

A Ministry of Justice spokesman said: "The easy availability of psychoactive substances in the community is an issue that compounds the difficulty of reducing supply in prisons.

"That is why we are working with agencies across the criminal justice system to address this problem.

"Training for staff on working with other local organisations - such as healthcare and the police - on how best to address the use of psychoactive substances is also being boosted to ensure we have a coordinated strategy on the ground to tackle these issues."